Field of the Invention
The present invention relates to methods and compositions for reducing the concentration of water soluble organics in water.
Background of the Related Art
Water soluble organics (WSOs) are organic molecules that occur naturally in crude oil. The particular combination and amount of WSO species are unique to the region of origin, and up to 120 WSO species can be present in the same crude oil sample at any given time. WSOs are polar compounds mainly comprised of medium chain carboxylic acids. These are sometimes also referred to as naphthenic acids. As their name implies, WSOs have a tendency to accumulate in the water phase, especially when pH is ≧6.0.
A majority of hydrocarbons are produced from subterranean formations along with significant amounts of water, referred to as produced water. An accumulation of WSOs in the produced water can become problematic, since production of the hydrocarbons cannot continue without a way to compliantly dispose of or reuse the produced water. Even though WSOs are usually present in the produced water at relatively low levels of about 1000 ppm, WSOs contribute to the amount of overall oil and grease. “Oil and grease”, or simply “O&G”, is a term for organic pollutants in water. Technically, O&G is the set of organic molecules that extract into n-hexane (hexane extractable material, HEM) from water at a pH of 2 or less, and remain as a residue after the n-hexane is boiled away. WSOs are the subset of O&G that, furthermore, adsorb onto silica gel, which may be referred to as “silica gel treated—hexane extractable material” or “SGT-HEM.” The amount of WSOs, or SGT-HEM, are measured indirectly as the difference between O&G, or HEM, and the material that does not adsorb onto silica gel.
FIG. 1 is a flow diagram of a method 10 for determining the amount of WSOs in water by filtering out the WSOs in one of two identical water samples. In step 12, two identical water samples are taken, and then one of those samples is filtered through a filter that is active for polar organics alone, such as a silica gel, in step 14. The unfiltered sample is processed according to the steps on the lefthand branch of the flow diagram, and the filtered sample is processed according to the similar steps on the righthand branch of the flow diagram. Accordingly, both the unfiltered and filtered samples are acidized (steps 16A, 16B); combined with an extractant, such as hexane, Vertrel or other approved solvent (steps 18A, 18B); analyze the extract for oil, such as by gravimetric analysis (weight difference) or infrared analysis (steps 20A, 20B); and determine the total oil and grease (Total O&G) in the unfiltered sample (step 22A) and the free oil and grease (Free O&G) in the filtered sample (step 22B). Finally, step 24 determines the amount of water soluble organics (WSO) as equal to the Total O&G less the Free O&G.
FIG. 2 is a flow diagram of an alternative method 30 for determining the amount of WSOs in water by adjusting the pH of one of two identical water samples. In step 32, two identical water samples are taken, and then an acid is added to one of those samples in step 34. The acidized sample is processed according to the steps on the lefthand branch of the flow diagram, and the unacidized sample is processed according to the similar steps on the righthand branch of the flow diagram. Accordingly, both the acidized sample (box 36A) and unacidized sample (box 36B) are combined with an extractant, such as hexane, Vertrel or other approved solvent (steps 38A, 38B); the extract is analyzed for oil, such as by gravimetric analysis (weight difference) or infrared analysis (steps 40A, 40B); and the total oil and grease (Total O&G) in the unfiltered sample (step 42A) and the free oil and grease (Free O&G) in the filtered sample is determined (step 42B). Finally, in step 44, the amount of water soluble organics (WSOs) is determined as the Total O&G minus the Free O&G.
Currently, limits set forth by the EPA (Methods 413.1/413.2 and 1664) allow produced water discharged into the Gulf of Mexico to contain a monthly average of no more than 29 mg O&G per liter of produced water. Daily discharge limits for O&G can be as high as 42 mg/L; however, higher discharges must be compensated for with discharges having correspondingly lower O&G concentrations, such that the average for the month is ≦29 mg/L.
Acids being the main constituents of WSOs accounts for the fact that WSO accumulation in water is largely governed by pH. Higher pH causes the acid/base equilibrium to shift towards the presence of water soluble carboxylate salts, while at lower pH, the absence of charge imparts enough lipophilicity to partition the molecules back into the oil phase. There, the acids form dimers via hydrogen bonds, causing them to become even more lipophilic and able to migrate further into the oil away from the water/oil interface. This clears the interface to make room for more protonated acids to enter the oil phase, thus becoming another driving force for the equilibrium. Although mechanical means such as filtration can be used to remove WSOs oftentimes more efficiently, chemical treatment is generally the most cost-effective method for WSO control. With regard to chemical treatment with conventional water clarifiers, WSOs remain even though dispersed oil is removed. This is because WSOs are mostly dissolved, not dispersed, in the system. Consequently, WSO levels are typically unaffected by conventional water clarifier treatment. Cationic water clarifiers typically, at best, remove from 10 to 20 percent (%) of the WSOs that may be associated with microemulsions.
The standard treatment for WSO control is simple acidification via mineral and/or organic acids. These types of acids are considered strong acids with a pKa of 3.2 or less. Disadvantages of acid treatment include handling hazards (health, safety, environmental) and space requirements (due to the large volumes needed). Furthermore, some acids pose corrosion and/or scale risks, and may interfere with the effectiveness of water clarifier treatment to remove dispersed oil. U.S. Pat. No. 5,354,477 involves the use of low molecular weight amines or amine quaternaries in combination with strong acid to remove water soluble organics. The weak, hydrophilic α-hydroxymonocarboxylic acids disclosed in U.S. Pat. No. 6,695,968 are a less severe option (pKa>3.8) that circumvents various disadvantages, but must be administered with anionic polymers for optimal WSO control.